markbradford

Xpost - Should I go with a Bailey or Forge dump valve and why?

hater

i like bailey, no problems at all

Audiboy

;P

Well, at least the dealer could blame a turbo failure for an aftermarket part related to turbo performance as I have read posted on the S4 forum. But exactly as to why they may even think of blaming the aftermarket part, other than the fact that it is aftermarket, could be some engineering also.

The stock valve has a very small dispalcement vacuum chamber, and with my mightly vac, I was able to open the valve and create a vacuum of 22 mmHg with 4 pumps of the mighty vac. On a bailey I installed about a month ago on an A4, it took me 7 pumps. My forged valve installed in the TT took 8 or 9 pumps. The vacuum chamber on the baileys and forged valve is pretty big compared to the stock one (pretty obvious).

So what does this do to performance? I am not entirely sure yet, but the turbo cars have a small vacuum reservoir on top of #3,4 cylinder (the black thing saying Danger! High Voltage! on it - actually that is for the coil packs, not the vacuum reservoir). The vacuum needed to run the BPV is controlled and isolated from manifold vacuum via a 1 way valve and a solenoid switch controlled by the ECU. This is true for DBW cars, I haven't seen the layout closely yet on a cable car. May be a direct connect all the way to vacuum.

Therefore, if there is a limited initial supply of vacuum from the reservoir, extra vacuum may be necessary to open a larger BPV. This means delay in opening the BPV - If it even opens at all during throttle modulation (like when you floor, let go, floor, let go the gas pedal on spirited driving.

Why did I care? Because initial driving of the TTR with the forged valve did show improved on-off-on throttle response, with some occasional surging as in sudden rush of power. While this may put smiles on a few drivers, I already pictured an overlean on-throttle condition because of premature residual boost that could damaged the engine. To alleviate this, I put back the stock ECU in the TTR, until I can satisfy myself that the BPV is reacting properly.

Why did I chose forged? Because I can open it and decrease the spring rate should I have any problems of it opening due to the larger volume content (something I was contemplating before the purchase of a BPV).

Another simple fix for above is to add a second vacuum reservoir (from an old VW) so that there is enough force (vacuum) in reserve when the ECU tells the solenoid valve to open or shut the BPV.

Anyone feel free to comment on this. The stock BPV was engineered for something, I want to make sure to keep it within the lines and now blow anything up. Other than that, the performance improvement was impressive - breaks tires easy even while in motion (1st, 2nd gear) wet weather though.

Psst, TT owners, I have performance tips for you that will only take a few minutes

Alf Garnett

I am always up for increases in performance/behaviour of this car...so do tell =)
Marcus

Audiboy

intercooler(s). Pull off lower left (and right) grill (by hand or non-scratching hook), take off that plastic piece snapped in that grill and there you go, instant larger opening for the intercoolers.
To get larger, get a blade and cut some of the plastic from the shroud. TT owners can easily see what I meant. I have pics but in 35mm since I was trying to use up the camera. I can take a digital shot tonight if anyone still confused.

Tip #2, Did you always wonder where the intake sucks it's air? Well it goes to the left fender behind the headlight. Did anyone ever open this part up? It is a tiny triangular tube that has the diameter about half the piping to the air meter and turbo. Restriction? Maybe - high speed. I do not like it at all. Easy fix, take it out. Need to take out the airbox, which is not too hard. Just unplug the hose off the MAF, unclip the PCV (oil vent hose), take off 2 philips screws to expose air filter, take off 2 10mm bolts to take off the air box and then take off the one extra bolt that holds this restrictor . Result, stock box with better breathing - and you will instantly hear the whoosh whoosh of the BPV.

My next mod, gotta vent in some of that outside air so that the intake box can breathe easier, and not from behind a headlight. While I'm at it, I'll install some brake ducting - all for Laguna Seca next month

LCP

...and even read a story of a TT owner that got AoA to reimburse him/her for Bailey valves on a TT. If a manufucturer is paying for a non-OEM replacement part on a car under warranty, I doubt there is much warranty concern, especially with the notion out there that these Bosch bpv's have been defective for their intended purpose ever since Audi started putting them in. The extra vacuum is interesting though. Is volume or pressure generated the key in figuring out when the valves will open though? I don't know enough about the vacuum system to say.

Audiboy

Given a fixed vacuum source - such as a small 1/4 inch hose with a max flow rate per unit time connected to manifold vacuum - then a large volume will need more time to be evacuated from atm pressure to reach max vacuum in equilibrium of what the engine can give. The BPV will open at a certain vacuum level depending on the size of the diaphragm and the spring rate. And, also depending on how it is installed, if the diaphragm (or piston) is facing the forced air side (reverse install?), whatever area is exposed to the forced air can weaken the effect of the spring return and help it force open faster.

Anyway, German cars are the predominant cars that I see use vacuum reservoirs because they utilize this "free" energy to power up a few things like valves, door locks, heater/AC components. The rest of the world just use one way valves to keep the vacuum on that side regardless of what's in the manifold such as the brake booster, cruise control, etc. The nice thing about having a vacuum reservoir, just like an air tank, is that once you have that system on vacuum, it will always be present as long as it is separated via a one way valve from manifold vacuum and there is no leak. Then, a solenoid will be needed to divert that vacuum to a vacuum motor somewhere. It this case, a BPV. Just like an air tank, if you have 40psi stored in a tank, and you have to inflate a tire from 20 psi, the end pressure is a differential of the 2 volumes as expressed by the PV=nRT equation.

Same sorta thing applies to vacuum. On one side of the vacuum control solenoid, you have a vacuum side with a reservoir of a given volume including the hoses (as long as they do not collapse and change the volume) sitting at around manifold vacuum. And then on the other side of the solenoid, you have a non pressurized BPV sitting at 1atm pressure - pressurized will be even worst to evacuate, hence no pressure on this side of the BPV. Then opening the solenoid will introduce a larger volume and pressure to the vacuum source and will stabilized at a certain vacuum. So here, you can kinda picture what a larger volume on the BPV side can do. However it doesn't really stop there because the vacuum side has a vacuum source from the 1 way valve to a tiny ¼ inch hose to manifold vacuum. If manifold vacuum is present due to close throttle, then eventually there will be enough vacuum to bring the whole thing in equilibrium. In the case of on-off-on throttle, this manifold vacuum source is erratic and the vacuum really depends on the reservoir. Hence why adding a reservoir will not really affect anything but just keep more "free" energy on tap.

I chose 22mmHg even if the valves appear to be somewhat fully open around 18mmHg because a car at idle and coasting typically produces about that much vacuum. So a larger BPV will require "more vacuum volume" to open but that also depends on the spring. The volume also collapses as the piston retracts, hence the movement is not linear, gets accelerated as the volume collapses. But the critical point is the first opening or cracking of the BPV, and that's where we may need a stronger vacuum source. The spring can be played with on the Forged to match the maximum force needed to keep the valve shut at max turbo pressure. Another quick fix is a second vacuum reservoir.

As far as service advisors OK'ing the use, could be I'm overreacting, but the surging on the TT is not overreacting. Their opinions may vary as I have seen on the board over the years as one denies a claim on one, and vice versa. I'm just trying to play it safe.

Well, off I go to play with the normally aspirated world. I'll play with the Nimbus a few days before Quattro Club. Damn, I've got the flu now - crap.

unA4dable

...if you consider the total volume of all of the vacuum hoses running around an A4, even if the vacuum chamber in the aftermarket valve has twice the volume of the stock part, the overall increase in volume to the vacuum system is gonna be pretty darned small. I doubt it'd have much, if any, effect.

-dan

Audiboy

Do you wanna do this check yourself? One way valves have a purpose you know.